T cells are activated and respond to specific peptide:MHC (pMHC) complexes. Understanding the mechanisms by which T cells are triggered is of critical importance if we are to rationally optimize the adaptive immune response to cancer and viral infections. In addition, discerning the ways pMHC complexes stimulate T cells will allow for immune-based therapies to regulate autoimmune disease. A consistent factor in the activation of T cell responses against tumors, viruses, and self antigens is the low stability of some pMHC complexes. The overall goal of this proposal is to characterize the mechanism determining the potency of less stable pMHC complexes and their ability to elicit T cell responses. The specific aims are designed to address the following questions: How do less stable pMHC complexes trigger T cell responses? Do less stable pMHC complexes alter effector functions? How can we optimize the response to these weaker ligands? Does T cell receptor interaction with pMHC change the stability of the complexes? Answers to these questions wilt broaden our overall understanding of T cell activation and specifically provide methods for optimizing response to less stable peptides typical of tumor associated and viral escape antigens.